The present invention generally relates to a measuring apparatus and a measuring method, and more particularly, to a measuring apparatus and a measuring method for measuring a polarization characteristic (birefringence) of an optical system (specifically, a projection optical system) of an exposure apparatus used during a lithography process in manufacturing various devices such as a semiconductor device and a liquid crystal display device.
In manufacturing very small devices with a photolithography technique, a reduced projection exposure apparatus has been used in which a projection optical system projects a circuit pattern onto a photosensitive substrate such as a wafer to transfer the circuit pattern to the photosensitive substrate.
The reduced projection exposure apparatus needs to transfer a reticle pattern accurately to a wafer at a predetermined scaling factor. To satisfy the need, it is important to use a projection optical system (a projection lens) excellent in optical performance such as an imaging property. In recent years, particularly, miniaturization of devices accomplished at a fast pace has often required pattern transfer beyond a normal imaging property, and the pattern transfer is becoming sensitive to birefringence which is a polarization characteristic of an optical system.
Several apparatuses and methods have conventionally been proposed for measuring birefringence. For example, a proposed apparatus measures birefringence by using light transmitted through a test object similarly to a stress measuring apparatus for a semiconductor wafer. Such an apparatus, however, can only measure birefringence at a certain point on a surface of the test object (in other words, it only can perform point measurement) since the apparatus has no image-forming system. When birefringence needs to be measured over a wide area, scanning should be performed for the test object (or for a light source). Thus, the apparatus is complicated and is not suitable for a test object which needs measurement over a wide area such as a projection optical system. To address this, another proposed apparatus enables measurement of birefringence of a test object over a wide area thereof by using a shearing interferometer having an image-forming system. For example, Japanese Patent Laid-Open NO. 2004-61515 has proposed such an apparatus.
The apparatus using the shearing interferometer proposed in Japanese Patent Laid-Open No. 2004-61515 measures birefringence with a diffraction grating removed, while the diffraction grating is inserted thereinto when a wavefront is measured. As a result, the apparatus is directly affected by birefringence in the diffraction grating, thereby making it impossible to measure the birefringence accurately.
In addition, diffracted light emerges in a direction at an angle with respect to the incident light, so that the direction of polarization can be changed. It is thus difficult to eliminate the influence of the optical system (that is, the image-forming system included in the shearing interferometer) other than the test object (such as a projection optical system) in measuring a wavefront, and it is impossible to provide complete consistency between the wavefront measurement and the birefringence measurement. In other words, the influence of the optical system included in the apparatus upon the polarization characteristic cannot be measured precisely, and the birefringence of the test object cannot be measured accurately.